[Subtask] Support policies for function metadata objects

Describe the subtask

Support associating policies with FUNCTION metadata objects through the existing metadata object policy REST APIs.

Currently, FUNCTION exists in MetadataObject.Type, but it is excluded from PolicyManager.SUPPORTED_METADATA_OBJECT_TYPES_FOR_POLICIES. As a result, policy association requests for functions are rejected with Cannot associate policies for unsupported metadata object type FUNCTION.

Implementation scope:

• Add FUNCTION to the supported metadata object types in PolicyManager.
• Clean up function policy relations when a function is dropped.
• Extend catalog and schema cascade cleanup for function policy relations across H2, MySQL, and PostgreSQL.
• Update docs/open-api/policies.yaml to include FUNCTION.
• Add unit and integration tests covering policy association, listing, retrieval, removal, direct function deletion, and cascade cleanup.

The completed function tag implementation in #11844 and #11897 can be used as a reference.

This task covers the server, core relational storage, OpenAPI, and their tests. Typed Java and Python client APIs can be handled in follow-up issues.

This task extends the current direct object-policy model and does not implement the policy-on-tag model proposed in #12176.

Parent issue

#6158